Digital watermarking refers to the embedding of a message in an image or image sequence for such purposes as establishing ownership, tracking the origin of the data, preventing unauthorized copying, or conveying additional information (meta-data) about the content. Watermarking has potential uses in a wide range of products, including digital still and video cameras, printers and other hardcopy output devices, and content delivery services (e.g., Internet-based photofinishing).
Numerous image watermarking methods have been described in the prior art, including both patents and the technical literature. Many of these methods are described in review papers such as: Hartung and Kutter, Multimedia Watermarking Techniques, Proc. IEEE, 87(7), pp. 1079-1107 (1999), and Wolfgang et al., Perceptual Watermarks for Digital Images and Video,” Proc. IEEE, 87(7), pp. 1108-1126 (1999).
A basic distinction between various methods is whether the watermark is applied in the spatial domain or the frequency domain. In either approach, many techniques use a pseudo-random (PN) sequence in the watermark generation and extraction processes. The PN sequence serves as a carrier signal, which is modulated by the original message data, resulting in dispersed message data (i.e., the watermark) that is distributed across a number of pixels in the image. A secret key (i.e., seed value) is commonly used in generating the PN sequence, and knowledge of the key is required to extract the watermark and the associated original message data.
The use of a PN sequence in the watermarking process typically results in a watermark signal that has similar visual characteristics to a random noise field. While noise is an inherent component of digital images because of sensor noise (whether originating from film grain when film is scanned or from sensor noise in a digital camera), it is generally desirable to minimize the additional noise that is introduced by the watermarking process. This is accomplished by reducing the amplitude of the watermark signal so that it is below the threshold of perception (i.e. subthreshold). The result is a watermark that is visually transparent, i.e. the watermark cannot be perceived by an observer under typical viewing conditions.
In some applications it may be impossible to provide sufficient robustness to the embedded watermark and still meet the goal of visual transparency. This is because a low amplitude watermark is more vulnerable to removal attacks, where an attempt is made to remove the watermark by using various types of image processing methods, e.g. lowpass filtering. In such cases, it becomes necessary to increase the watermark amplitude, which may result in a watermark pattern that is easily detectable (i.e. suprathreshold), and perhaps even objectionable, when viewed by an observer.
There are watermarking techniques that intentionally use suprathreshold watermarks. Some of these techniques embed watermarks that are easily visible to any observer, e.g., a logo that is placed in the corner of a television screen to indicate the broadcasting station. These types of watermarks are called obvious watermarks. Other suprathreshold watermark techniques are more subtle in that the watermark may be obvious to an observer who knows the nature of the watermark, but it is not obvious to an uninformed observer. An example of this latter approach can be found in U.S. Pat. No. 5,699,427 issued Dec. 16, 1997 to Chow et al. entitled Method to Deter Document and Intellectual Property Piracy Through Individualization. This patent teaches a method for modifying documents and images in such a way that numerous variations of the original can be produced, but these variations do not change the overall meaning or usefulness of the documents or images. The variations can include changing the spelling of words in a document or the color of objects in an image. A limitation of this approach is that the amount of embedded information is constrained, and it is also very difficult to embed arbitrary information such as a time/date stamp. Moreover, the method requires significant manual intervention to identify the words, objects, etc. that can be modified without affecting the overall meaning or usefulness of the document or image.
There is a need therefore to have a watermarking technique for images that allows the embedding of a high amplitude watermark in such a way that: 1) the resulting watermark is not objectionable to an observer; 2) arbitrary information can be included in the watermark; and 3) manual intervention is not required to perform the embedding process.